The present invention relates to a method for preparing inorganic ion exchangers, catalysts, getters, dielectric materials and ceramic materials. In particular, the present invention is directed to a method for preparing gels, including electrophoresis gels and spherules of hydrous iron oxide and variations thereof. The hydrous iron oxide gels are prepared using an internal gelation process through the implementation of process control parameters that control the type of gel, gel shape and size, as well as microstructure of the material.
The present application in part discloses material contained in U.S. Pat. No. 6,492,016, issued Dec. 10, 2002, entitled xe2x80x9cMethod for Preparing Spherical Ferrite Beads and Use Thereof,xe2x80x9d by Robert J. Lauf et al., filed on even date herewith, the entirety of which is incorporated herein by reference.
Hydrated oxides of many metals (such as titanium, zirconium, iron, hafnium, tin, aluminum, lead, cerium, tungsten, magnesium, manganese, etc.), acidic salts of polyvalent metals (phosphates, tungstates, antimonates, molybdates, tellurates, selenates, silicates, vanadates and hexacyanoferrates of elements such as ammonium, titanium, zirconium, hafnium, tin, lead, etc.), and heteropoly acid salts (ammonium molybdophosphate, ammonium phosphotungstate, ammonium molybdosilicate, ammonium tungstoarsenate, titanium phosphosilicate, etc.) are very effective inorganic ion exchange materials. Because inorganic ion exchangers are stable in high radiation fields, they are especially important in the removal of radionuclides from waste streams. They have high selectivities and efficiencies for separating and removing fission products (e.g., cesium, europium, cerium, ruthenium, zirconium, and strontium), actinides, and other elements (such as silver, lead, mercury, nickel, zinc, chromium, and fluoride) from aqueous waste streams. Most of these materials are also compatible with the matrices used for long term waste storage such as in glass, phosphate or grout. Certain metal oxides, such as iron oxide and titanium oxide, are known to be effective for use in the photocatalytic decomposition of various hazardous organics and for many other catalytic purposes. Also, many metal oxides are known to be very effective as getters in removing volatile fission products from off-gas streams over a broad range of temperature. As used herein, the term xe2x80x9cgettersxe2x80x9d is meant to include any material capable of trapping another material within the getter material. For example, quartz wool (SiO2) is used to remove volatile radioactive cesium from the off-gas stream of gas-cooled nuclear reactors in Great Britain.
Inorganic exchangers and sorbents, such as hydrous iron oxide, are only commercially available as pure material in powder or granular form. These fine powders and granular particles are not readily adaptable to continuous processing, such as column chromatography. They have poor hydrodynamic properties. Some of these powders are also made as pellets by using binding materials; however, the binders tend to lessen the number of exchange sites that are available for use. The binders also tend to block pores and passageways to the exchange sites within the structures and can adversely affect the loading and kinetic behavior of the exchangers.
Another disadvantage of many of the powders, granular material, and pellets is lack of sorbent reproducibility of the inorganic ion exchangers. These materials are prepared in batch processes in which chemical and physical gradients can occur that cause variances in the crystal morphology and compositions of the products. Also, the granular material is not very stable and tends to powder or erode, causing problems in column operations. Pelletized hydrous iron oxide that is held together by binding material can be used in columns; however, the loading capacity of this material is lower. Additionally, organic binders, when used to make the pellets, are not stable when exposed to high radiation doses. Finally, resins that contain hydrous iron oxide particles have less capacity for loading and are not stable when exposed to high radiation.
Inorganic exchangers have also been made by taking fine particles of hydrous iron oxide and embedding therein organic resins or inorganic materials, such as asbestos or zeolites. However, these embedded particles suffer from the same disadvantages as the other particle and granular-based ion exchange materials.
Individuals have attempted to remedy the problems associated with powders and particles by forming gel particles. There are a number of gel forming processes used in the preparation of inorganic sorbents, catalysts, ceramic materials, dielectric materials, and getters. Common to all these processes is that the constituents of the processes need to be suitable for the bonding of colloidal particles into gel structures. The gels usually are hydrous metal oxides. These processes are generally identified as xe2x80x9csol-gelxe2x80x9d processes and the chemistries are complex and path dependent. Typically, they are defined as external or internal gelation processes. In the external gelation processes, gelation reactions involve mass transfer to a second phase or fluid. By comparison, there is little or no mass transfer in the internal gelation processes.
One of the original external gel processes for the preparation of nuclear fuels was developed at Oak Ridge National Laboratories (hereinafter ORNL). It was based on the gelation of colloidal sol droplets by extracting the water from them in an immiscible alcohol. In other external gelation processes developed at various European laboratories, droplets of solutions of organic polymers or sols were chemically gelled with ammonia, usually by mass transfer of the ammonia from a surrounding gas or solution.
Making silica-alumina gel as spheres is an example of one internal gelation process. Gel spheres were made by continuously mixing an acid solution of AlCl3 or Al2(SO4)3 with sodium silicate as drops into an immiscible organic medium. The aqueous droplets gelled while in the organic medium. The key to this process was the slow or delayed gelation of silica when the sodium silicate was acidified.
The most widely studied internal gelation processes in recent years involve the water hydrolysis of metal alkoxides. In these processes, solution temperature and pH are key parameters used in controlling hydrolysis and polymerization. However, materials made by the metal alkoxide processes typically are fine powders. Additionally, due to the complex chemistries involved and the difficulty in operating the process, it was difficult to form gel-spheres of hydrous metal oxides wherein the reaction could be controlled and the final product was predictable.
Accordingly, what is needed is a method of forming a hydrous metal oxide gel, specifically a hydrous iron oxide gel, wherein the gel is effective as an inorganic sorbent, catalyst, ceramic material or getter. What is also needed is a method of forming a hydrous metal oxide gel wherein the characteristics of the gel may be controlled to provide a gel that is useful for a variety of different uses. Finally, what is needed is a method of forming a hydrous metal oxide gel wherein the metal oxide gel may include other constituents that are selected to remove a variety of different materials, thereby increasing the usefulness of the metal oxide gel.
Accordingly, it is an object of the present invention to provide new methods for preparing inorganic ion exchangers and sorbents into a more useful form.
It is another object of the present invention to provide new methods for preparing more useful forms of catalysts.
Yet another object of the present invention is to provide new methods for preparing more useful forms of dielectric materials.
Still yet another object of the present invention is to provide new methods for preparing more useful forms of getters.
Another object of the present invention is to provide new methods for preparing more useful forms of ceramic materials.
Still another object of the present invention is to provide new methods for preparing gels for use in capillary, film or slab gel electrophoresis.
It is still yet another object of the present invention to provide new methods for creating more surface area in hydrous iron oxide gels.
Another object of the present invention is to provide new methods for forming macroporous iron oxide spherules.
Yet another object of the present invention is to provide new methods for converting hydrous iron oxide spherules to other chemical forms, including, but not limited to, phosphates, tungstate, molybdate, vanadate, and selenate.
Still another object of the present invention is to provide new methods for making ultra fine hydrous iron oxide particles using an electric dispersion reactor (EDR).
Another object of the present invention is to provide spherules of hydrous iron oxide and variations thereof that are used as inorganic ion exchangers.
It is still another object of the present invention to provide spherules of hydrous iron oxide and variations thereof that are used as catalysts.
Yet another object of the present invention is to provide spherules of hydrous iron oxide and variations thereof that are used as getters.
It is still yet another object of the present invention to hydrothermally convert hydrous iron oxide spherules to ferrite spherules of barium, strontium, and lead and mixtures thereof, which have catalytic and dielectric properties that are useful in the catalyst and the electronic industries
Another object of the present invention is to provide new inorganic ion exchangers as spherules that exhibit good chemical stability in acidic and basic solutions.
Still another object of the present invention is to provide new inorganic ion exchangers as microspherules that are highly selective for certain cations and anions.
Yet another object of the present invention is to provide new inorganic ion exchangers as microspherules that are compatible with final waste forms.
It is still another object of the present invention to provide new inorganic ion exchangers as microspherules that improve the flow dynamics for column operations.
These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method for preparing hydrous iron oxide gels comprising the steps of first forming a stable homogeneous aqueous broth comprising HMTA, urea and an iron metal salt. The broth has an HMTA concentration in a range from about 0.4 M to about 2.5 M, a urea concentration in a range from about 0.4 M to about 4.0 M, and an iron metal salt concentration in a range from about 0.4 M to about 2.2 M. Then, the aqueous broth is placed within a gel-forming operation, wherein the gel-forming operation is at a temperature from about ambient to about 100xc2x0 C. The temperature of the gel-forming operation is maintained from about ambient to about 100xc2x0 C. to form a hydrous iron oxide gel using an internal gelation process.
In accordance with another aspect of the present invention, other objects are achieved by a method for preparing hydrous iron oxide gel comprising the steps of first forming a homogeneous aqueous broth comprising HMTA, urea, an iron metal salt, wherein the broth is at a temperature from about 0xc2x0 C. to about 10xc2x0 C., and wherein the broth has an HMTA concentration in a range from about 0.4 M to about 2.2 M, a urea concentration in a range from about 0.4 M to about 4.0 M, an iron metal salt concentration in a range from about 0.4 M to about 2.2 M with the iron being partially hydrolyzed with ammonium hydroxide to provide an OHxe2x88x92/Fe+3 mole ratio of 0.0 to 1.5, further wherein said broth has a mole ratio of HMTA to iron greater than 0.75:1 and a mole ratio of urea to iron greater than 0.75:1. Then, placing the aqueous broth within a gel-forming operation, wherein said gel-forming operation is at a temperature from about 45xc2x0 C. to about 95xc2x0 C.; and then maintaining the temperature of the gel-forming operation from 45xc2x0 C. to about 95xc2x0 C. to form a hydrous iron oxide gel using an internal gelation process.